CSCI 2210: Programming in Lisp

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CSCI 2210 Programming in Lisp

• Introduction to Lisp

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CSCI 2210 Programming in Lisp

• Instructor
• Alok Mehta (mehtaa_at_cs.rpi.edu)
• Office Hours After class or by appointment
• Home Phone [phone]
• Teaching Assistant
• Course Web Page
• http//www.cs.rpi.edu/courses/spring99/lisp
• Textbook
• Ansi Common Lisp, by Paul Graham Prentice Hall
  1996

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Syllabus
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Assignments/Grading

• Assignments/Grading
• HW 1 - Due on January 27 (25)
• HW 2 - Due on February 10 (25)
• HW 3 - Due on March 5 (25)
• Final Exam - March 3 (25)
• Homeworks
• Due at 115959 pm on the due date
• Total of two grace days are given for late
  submission
• After this, there is a late penalty of 10 points
  per day

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What is Lisp?

• Stands for LISt Processing
• An old, different programming language
• Why study Lisp?
• Different, but not outdated
• Simple, elegant syntax and constructs are
  extremely simple
• Encourages "bottom-up" software development
• Helps make computers intelligent
• Easy to write a program that write programs
• Programs that can modify themselves based on new
  knowledge
• Artificial Intelligence (AI) uses

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Examples of Lisp Applications

• Some popular programs written in Lisp
• Emacs
• Autocad
• Interleaf
• Other, misc. application areas
• Expert Problem Solvers (e.g. Calculus, Geometry,
  etc.)
• Reasoning, Knowledge Representation
• Learning
• Education
• Intelligent support systems
• Natural Language interfaces
• Speech
• Vision

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Lisp is Interactive

• Lisp is interactive
• Toplevel - when lisp is waiting for input
• Lisp expressions can be typed into the toplevel
• An expression typed into the toplevel is
  evaluated by the Lisp interpreter

gt 1 1 gt
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Atoms and Lists

• Atoms
• A single element of a particular data type
• Examples
• 1
• 3.3
• "This is a String"
• ThisIsASymbol
• Lists
• Lists may contain atoms or other lists
• Examples
• (1 8.1 3)
• (a b (c d) e)
• (73 22 b ("A" x) ((((3.14) (4 7) nil (32)))))

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Expressions

• Expressions
• Either atoms or a list
• Can represent expressions to be evaluated
• Format
• (operator arg1 arg2 )
• Zero or more arguments
• Example
• gt (sqrt 4.0) Take the square root of 4.0
• 2.0
• Can represent data and data structures
• (course CSCI221001
• (name (Programming in Lisp))
• (instructor
• (name (Alok Mehta))
• (email (mehtaa_at_cs.rpi.edu)))
• (department (Computer Science)))

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Expressions are evaluated

• We said
• Lisp expressions can be typed into the toplevel
• An expression typed into the toplevel is
  evaluated by the Lisp interpreter
• Everything (DATA PROGRAMS) in Lisp is an
  expression (i.e. an atom or a list)

gt 1 1 gt (sqrt 4.0) 2.0 gt ( 2 3) Note that
expressions are in prefix notation! 5
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Rules for Evaluation

• An expression is evaluated as follows
• Each argument are evaluated, from left to right
• Call by value occurs for the given operator
• Example gt (/ ( 4 6) 3)
• / is the operator skip this for now
• ( 4 6) is the first argument. Evaluate this
  (note the recursion here)
• is the operator skip this for now
• 4 is the first argument it evaluates to
  itself (4)
• 6 is the second argument it evaluates to
  itself (6)
• is now called, with arguments 4 and 6
  passed by value
• This evaluates to 24
• We currently have (/ 24 3)
• The second argument to / is 3, this evaluates to
  itself (3)
• / is called with arguments 24 and 3 passed by
  value
• (/ 24 3) is replaced by the value 8, which is
  returned to the toplevel

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More Expression Examples
gt (/ ( 4 6) 3)8gt ( 1 2 3 4 5) The
operator takes zero or more args15gt ()0gt
"Hello""Hello"gt (sqrt ( 1 3))2.0gt (quote (
1 3)) Exception! Quote Don't evaluate arg(
1 3)gt '( 1 3) shortcut for (quote ( 1
3))( 1 3)
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The Quote Operator

• Special Operator
• Disobeys the evaluation rule
• Does NOT evaluate its argument

gt (sqrt ( 1 3))2.0gt (quote ( 1 3))( 1 3)gt
(quote hello)HELLOgt 'helloHELLOgt helloError
Attempt to take the value of the unbound variable
HELLO'. condition type UNBOUND-VARIABLE
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Symbols

• In the expression below, HELLO is a Symbol
• gt 'hello
• HELLO
• Symbols are used for variables (and other things)
• The following expression causes the interpreter
  to search for a value for the symbol HELLO
• gt hello
• Error condition type UNBOUND-VARIABLE
• If the symbol HELLO was bound to something, the
  value would have been returned
• gt (set (quote hello) 3) initialize HELLO to 3
• 3
• gt hello
• 3

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Set, Setf Assign Variables

• Set and Setf assign variables (side effect)
• gt (set (quote a) '( 5 3))
• ( 5 3)
• gt (setf b ( 5 3)) equivalent to (set (quote b)
  ( 5 3))
• 8
• Examining variables
• gt a
• ( 5 3)
• gt b
• 8
• Accessing variables
• gt ( 3 b)
• 11
• gt ( 3 'b)
• error
• gt ( 3 a)
• error

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Overview of Lisp Syntax

• Overview of Lisp Syntax
• ( Left Parenthesis. Begins a list of items.
  Lists may be nested.
• ) Right Parenthesis. Ends a list of items.
• ( ( 3 2) ( 7 8))
• Semicolon. Begins a comment (terminates at
  end of line)
• ( ( 3 2) ( 7 8)) Evaluate ((32)(78))
• " Double Quote. Surrounds character strings.
• "This is a thirty-nine character string."
• Single (Forward) Quote. Dont evaluate next
  expression
• '(Programming in Lisp)
• Examples
• ( 3 2) returns the string "( 3 2) as an
  atom
• ( 3 2) evaluates ( 3 2) and returns 5
• '( 3 2) returns the expression ( 3 2) as a
  list
• Lisp is case-insensitive

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The Story So Far...

• Atoms
• Lists
• Expressions
• The Evaluation Rule
• Symbols
• Set, Setf
• Quote (')

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How to Run Lisp

• Under UNIX (on RCS)
• kcl, gcl
• Specify in homework which was used
• q to recover from an error
• D to exit
• Under Win '95
• Go to http//www.franz.com/dload/dload.html
• Select Allegro CL Lite for Windows

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Using Lisp on RCS

• Conventions
• UNIX Prompt
• gt LISP Interpreter prompt
• From a UNIX prompt, start the lisp interpreter
• gcl
• GCL (GNU Common Lisp) Version(2.2) Mon Sep 30
  094544 EDT 1996
• Licensed under GNU Public Library License
• Contains Enhancements by W. Schelter
• gt
• At the Lisp prompt, type your Lisp Expressions
• gt ( ( 3 2) ( 7 8))
• 75
• gt
• Lisp expressions return values
• Return values can be used in other expressions

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Using Lisp on RCS

• Recovering from errors in GCL (q)
• gt ( 4 x)
• Error "x" is not of type NUMBER.
• Fast links are on do (siuse-fast-links nil)
  for debugging
• Error signalled by .
• Broken at . Type H for Help.
• gtgt q
• Executing lisp commands from a file
• gt (load "prog1.lsp")
• Reads and executes the lisp expressions
  contained in prog1.lsp
• Accessing on-line help
• gt (help)
• Exiting from GCL (bye) or CTRL-d
• gt (bye)

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How to Write Lisp

• Experiment by entering expressions at the
  toplevel
• or, Write code into a file and save it
• Lisp code can be messy
• Use an editor with paren matching!
• vi set sm
• emacs M-x lisp-mode
• Load
• Reads a file and executes expressions contained
  in the file, as if you typed them directly at the
  top level
• gt (load "hello.lsp")
• t
• Load returns the result of the LAST expression
  evaluated in the lisp file

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Constructing New Lists

• One way Use quote operator
• gt (quote (1 2 3))
• (1 2 3)
• gt '(1 2 3)
• (1 2 3)
• Another way Use the list operator
• gt (list 1 2 3)
• (1 2 3)
• When is one better than another? Depends...
• gt '(1 2 (3 (4)) 5)
• (1 2 (3 (4)) 5)
• gt (list 1 2 (list 3 (list 4)) 5)
• (1 2 (3 (4)) 5)
• gt '(1 2 ( 3 4) 5)
• (1 2 ( 3 4) 5)
• gt (list 1 2 ( 3 4) 5)
• (1 2 7 5)

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Cons, Remove, First, Rest

• Lists are used to represent knowledge
• gt (setf complang '(C Lisp Java Cobol))
• (C LISP JAVA COBOL)
• Cons (CONStruct) adds an element to a list
• gt (setf complang (cons 'Perl complang))
• (PERL C LISP JAVA COBOL)
• Remove removes an element from a list
• gt (setf complang (remove 'Cobol complang))
• (PERL C LISP JAVA)
• First gets the first element of a list
• gt (first complang)
• PERL
• Rest gets everything except the first element
• gt (rest complang)
• (C LISP JAVA)

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Lists are like boxes NILEmpty
G H
C D
F
A B
E
I
J

• Lists are like boxes they can be nested
• ((( A B ) C D ( E ) ( )) ( F ) G H (((I)(J))))
• NIL is an empty list
• gt (setf messy '(((A B) C D (E) ( )) (F) G H
  (((I)(J)))) )
• (((A B) C D (E) NIL) (F) G H (((I)(J))))
• gt (first messy)
• ((A B) C D (E) NIL)

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First, Rest Revisited

• First returns the first element of a list
• Returns an atom if the first element is an atom
• Returns a list if the first element is a list
• Rest returns all elements of a list except the
  first
• Always returns a list
• Examples
• gt (first '((a) b)) returns (A)
• gt (first '(a b)) returns A
• gt (first '(a)) returns A
• gt (first '( )) returns NIL
• gt (rest '((a) b)) returns (B)
• gt (rest '(a b)) returns (B)
• gt (rest '(a)) returns NIL
• gt (rest '( )) returns NIL

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Getting the second element

• Use combinations of first and rest
• gt (setf abcd '(a b c d))
• (A B C D)
• gt (first (rest abcd))
• B
• gt (first '(rest abcd))
• REST Quote stops expression from being
  evaluated!
• Or, use second
• gt (second abcd)
• B
• third, fourth, , tenth are also defined

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Exercises

• Evaluate
• gt (first '((a b) (c d)))
• gt (first (rest (first '((a b) (c d)))))
• Use First and Rest to get the symbol PEAR
• (apple orange pear grape)
• ((apple orange) (pear grapefruit))
• (apple (orange) ((pear)) (((grapefruit))))

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Setf Revisited

• Setf
• Format
• (setf ltvar1gt ltvalue1gt ltvar2gt ltvalue2gt )
• Example
• gt (setf x 0 y 0 z 2)
• 2
• Returns
• the value of the last element
• Side effects
• assigns values for symbols (or variables) ltvar1gt,
  ltvar2gt,
• the symbol then becomes an atom that evaluates
  the value assigned to it
• gt x
• 0

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List storage

• Draw List Storage Diagram for
• (setf alist '(A B C))
• Explain semantics of functions
• first, rest, cons, remove
• Draw List Storage diagram for
• (apple (orange pear) grapefruit)

A B C
alist
Contents of Address Register (CAR) Old name for
First
Contents of Decrement portion of Register (CDR)
Old name for Rest
C
B
A
alist
Cons Cell
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Append, List

• Append
• Combines the elements of lists
• gt (append (a b c) (d e f))
• (A B C D E F)
• List
• Creates a new list from its arguments
• gt (list a b (c))
• (A B (C))

A
B
C
D
E
F
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Cons, Setf Push Pop

• Cons has no side effects
• gt (setf complang '(C Lisp Java Cobol))
• (C LISP JAVA COBOL)
• gt (cons Perl complang)
• (PERL C LISP JAVA COBOL)
• gt complang
• (C LISP JAVA COBOL)
• gt (setf complang (cons Perl complang))
• (PERL C LISP JAVA COBOL)
• gt complang
• (PERL C LISP JAVA COBOL)
• Push/Pop - Implement a stack data structure
• Push - shortcut for adding elements permanently
• Pop - shortcut for removing elements permanently
• gt (push complang Fortran)
• (FORTRAN PERL C LISP JAVA COBOL)
• gt (pop complang)
• (PERL C LISP JAVA COBOL)

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T, NIL

• Lisp uses the symbols T and NIL for boolean
  values
• T True
• NIL False (also means Empty List)
• These are reserved symbols and can't be
  re-assigned
• Note that NIL is used to mean both an empty list
  and a false
• gt 'nil
• NIL
• gt nil
• NIL
• The symbol NIL evaluates to itself
• gt '()
• NIL
• Functions that determine truth are called
  predicates

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Predicates listp

• Listp Is the argument a list?
• gt (listp 'Hello)
• NIL
• gt (listp '(1 2 3))
• T
• gt (listp nil)
• T
• NIL is a list with zero elements, but is still
  considered a list!
• gt (listp '())
• T
• gt (listp '(HELLO))
• T
• gt (listp (HELLO))
• Error
• gt (listp '( 1 2 3))
• T
• gt (listp ( 1 2 3))
• NIL

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Predicates null, not

• Null Is the argument an empty list?
• gt (null nil)
• T
• gt (null t)
• NIL
• gt (null 3.4)
• NIL
• gt (null '(1 2 3))
• NIL
• Not Returns the opposite of argument
• gt (not nil)
• T
• gt (not t)
• NIL
• gt (not '(1 2 3))
• NIL
• Not and Null are equivalent

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If

• If is a macro
• Macros do not follow the Lisp evaluation rule
• Syntax (if ltpredicategt ltthen-partgt ltelse-partgt)
• Evaluates the first argument, ltpredicategt
• If the first argument is true, the second
  argument is evaluated
• Otherwise, if the first argument is NIL, the
  third argument is evaluated
• Example
• (if (gt a b) a b) returns maximum of a and b

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And/Or

• And/Or
• Macros (don't follow Lisp evaluation rule)
• Lazy evaluation
• stop evaluating once you know the answer
• may not evaluate all arguments
• Return the value of the LAST argument evaluated
• And
• Returns true if all arguments are true stops at
  first false statement
• Or
• Returns true if any argument is true stops at
  first true statement

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Procedure Declaration

• Simple procedure in C/C to square a number
• double sqr (double x)
• return x x
• Equivalent function in Lisp
• (defun sqr (x) ( x x))
• Note
• C/C implementation is strongly typed Lisp is
  not
• Value returned by procedure is the last
  expression evaluated
• (defun sqr (x)
• ( 2 3)
• ( 7 8)
• ( x 3)
• ( x x))

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Defun

• Defun is a Lisp macro for DEfining FUNctions
• (defun ltproc-namegt
• (ltparameter1gt ltparameter2gt ...)
• ltexpression1gt ltexpression2gt ...)
• Side effect
• defines a user-defined lisp procedure
• Returns the name of the procedure defined
• Defun does not evaluate its arguments
• Resulting user-defined procedure is used like any
  other Lisp procedure
• gt (defun sqr (x) ( x x))
• SQR
• gt (sqr 5)
• 25

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Output

• So far the toplevel prints return values
• But, need a general way to print
• E.g. the load procedure only prints the return
  value of the last expression
• Print, Format
• gt (print 3)
• 3
• 3
• gt (defun verbose-add (a b)
• (format t "A plus A equals A." a b ( a
  b))
• ( a b))
• gt (verbose-add 3 5)
• 3 plus 5 equal 8
• 8

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Input

• Read
• gt (defun askem (prompt)
• (format t "A" prompt)
• (read))
• ASKEM
• gt (askem "How old are you? ")
• How old are you? 3
• 3

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C/C Example

• C/C Convert units of sq. meters to sq. yards
• include ltiostream.hgt
• void main ()
• const float meters_to_yards 1.196
• float size_in_sqmeters
• float size_in_sqyards
• cout ltlt "Enter size in square meters "
• cin gtgt size_in_sqmeters
• size_in_sqyards meters_to_yards
  size_in_sqmeters
• coutltlt"The size in square yards is "
• ltlt size_in_sqyards ltlt endl

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Equivalent Lisp Example

• Literal Translation
• gt (defun converter ()
• (setf meters_to_yards 1.196)
• (format t "Enter size in square meters ")
• (setf size_in_sqmeters (read))
• (setf size_in_sqyards
• ( meters_to_yards size_in_sqmeters))
• (format t "The size in square yards is
  A"
• size_in_sqyards)
• )
• CONVERTER
• gt (converter)
• Enter size in square meters 2.0
• The size in square yards is 2.392
• NIL

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Better Lisp Function

• Using a more lisp-like style of programming
• gt (defun converter (sq_meters) ( sq_meters
  1.196))
• CONVERTER
• gt (converter 2.0)
• 2.392
• Take advantage of toplevel
• Users enter expressions toplevel prints the
  return value
• Functional programming
• Avoid variable assignments, side effects
• This course covers the Lisp programming language
• Programming language constructs, built-in
  functions
• Lisp style, philosophy

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Review

• Lisp List Processing
• Data and Programs represented using Symbolic
  Expressions
• Atoms, Lists (represented using box analogy or
  cons cells)
• Interpreter functions (load, help, bye)
• Assigning variables (set, setf)
• List manipulation
• cons, remove, first, rest, append, list
• push, pop
• second, third, , tenth
• T, NIL, Predicates
• If, and, or
• Defun